1. Field of the Invention
This invention relates to a signal detector using a scanning probe and also to a probe microscope using such a signal detector.
2. Related Background Art
Since the invention of scanning tunneling microscope (STM) that allows the observer to directly observe the electronic structure of a conductor, microscopes adapted to acquire various pieces of information and their distribution patterns from an object have been developed in recent years. With such microscopes, information is obtained by scanning the object by means of a pointed probe. Such microscopes include atomic force microscopes (AFMs), scanning capacity microscopes (SCaMs) and near field optical microscopes (SNOMs). At present these microscopes are collectively referred to as scanning probe microscopes (SPMs) and widely used as means for observing microstructures with a resolution of the level of atoms and molecules.
An AFM is a microscope adapted to observe micro-undulations on the surface of a specimen by detecting the warp of a probe produced by atomic force. AFMs provide a wide scope of application because the AFM allows observing an insulator without problem unlike the STM through which only a conductor can be observed. Thus, they are attracting attention as they can be used for atomic/molecular manipulators of the next generation. A number of reports have been made on them.
Among others, non-contact atomic force microscopes (ncAFMs) adapted to observe the surface profile of a specimen in a non-contact region (attractive force region) without any physical contact between the front end of the probe and the surface of the specimen are known. The ncAFM is designed to oscillate the probe at a resonance point and detect the amount of shift of the resonance frequency of the probe due to the physical interaction between the surface of the specimen and the probe tip (atomic force and molecular force between the probe tip and the specimen surface) so as to allow observation of the surface profile of the specimen. Since the observation using an ncAFM is conducted in a non-contact region, any adverse effect of contact of the probe tip and the specimen surface can be avoided. For this reason, a broader application of ncAFMs as atomic and molecular manipulators is expected than ever.
In the ncAFM, the signal obtained by the probe is a signal subjected to frequency modulation. The reference frequency is the resonance frequency of the probe and the modulation, or the frequency shift xcex94f, represents the obtained surface information.
The FM detection technology using a PLL (phase locked loop) is widely used as a technology for detecting the amount of frequency shift (Shinlichi Kitamura and Masashi Iwasaki; Appl. Phys. Lett., Vol. 72, No. 24, 15 June 1998).
A circuit adapted to receive a signal subjected to a frequency shift as input signal in a detection system using a PLL, generate a reference signal having a frequency same as the resonance frequency of the probe in the detection system, detect the phase difference between the input signal and the reference signal and convert the phase difference into a voltage is known.
A phase delay occurs when the frequency of the input signal is lower than that of the reference signal, whereas a phase advance takes place when the frequency of the input signal is higher than the frequency of the reference signal. Therefore, the output of the detection system relative to the frequency of the input signal shows a voltage change before and after the reference signal frequency f0 as shown in FIG. 7 of the accompanying drawings. The width of the change between fun and fox in FIG. 7 is determined by the reference signal frequency located at the middle of the frequency change due to the principle of detection of phase difference. Therefore, when the reference signal frequency is high, both fmin and fmax become high accordingly. Thus, the expected amplitude of the detection signal when the width of modulation of the input signal is xcex94f is substantially equal to the value determined by the ratio of the amount of frequency shift xcex94f relative to the reference signal frequency located at the middle (xcex94f/f0)
Meanwhile, in the ncAFM, the probe is oscillated at the resonance point of the probe and the amount of frequency shift is detected at the resonance point for the observation of the surface of the specimen. While a frequency between several times of 10 kHz and several times of 100 kHz is popularly used for the resonance frequency of the probe, a probe having a relatively high resonance frequency is popularly used for the purpose of raising the scanning frequency to be used for observation and minimizing the influence of external noises. However, the amount of frequency shift of the resonance frequency that is detected as a signal representing the surface profile of the specimen is between several Hz and several times of 100 Hz and hence very small if compared with the resonance frequency of the probe. For this reason, a highly sensitive detection system is required for detecting the fluctuations of such a small amount of frequency shift. Additionally, when a PLL is used for detecting the fluctuations of the frequency, the frequency stability of the VCO (voltage control oscillator) becomes a problem, particularly a serious noise problem, when detecting such small frequency fluctuations are to be detected. Furthermore, the output frequency of the ncAFM for the input control voltage of the VCO can, if partly, not necessarily be linear. If the shift of the resonance point of the probe is detected in the part that is not linear, the image obtained as a result of the observation may not correctly reflect the surface profile of the specimen.
For detecting a frequency signal with such a high sensitivity, a large output value may be obtained relative to the frequency fluctuations that are input to the detection system by increasing the inclination of the graph of FIG. 7. Ideally, inclination is so regulated as to be able to obtain Vmax (maximum output voltage of the detection system) for the amount of frequency shift xcex94f by regulating the resonance frequency of the probe to the fo point. However, as pointed out earlier, the resonance frequency of the probe is between several times of 10K and several times of 100 KHz and can vary from probe to probe even among the probes prepared through a same process. The variance is significantly larger than the amount of frequency shift of the resonance point. Therefore, if the probe is replaced and the detection system is used to detect signals without being regulated for the new probe, it may sometimes be impossible to detect the amount of shift of the resonance point because the f0 point is shifted to allow the signal to overflow. For this reason, the efficiency of the operation of replacing the probe has been poor because the replacement requires the values of the elements of the circuits of the detection system that have been regulated before to be changed and regulated for another time.
In view of the above described circumstances, it is therefore the object of the present invention to dissolve the above identified problems by providing a signal detector comprising a scanning probe that can raise the ratio of the amount of frequency shift relative to the resonance frequency of the probe and can accommodate variance of the resonance frequency of different probes. Another object to the present invention is to provide a probe microscope using such a signal detector.
In an aspect of the invention, the first object of the invention is achieved by providing a signal detector comprising:
a frequency changing circuit adapted to receive an electric signal having a frequency modulated from a first reference frequency f1 with a modulation width xcex94f as input, convert the received electric signal to an electric signal having a second reference frequency f2 lower than the reference frequency f1 and output the converted electric signal; and
a frequency/voltage conversion circuit adapted to receive the output of said frequency changing circuit as input and output a voltage signal corresponding to the ratio of the modulation width xcex94f to the second reference frequency f2 and an electric signal having a frequency modulated from a reference frequency Nf2 with a modulation width of Nxcex94f (N: integer).
In another aspect of the invention, there is provided a signal detector using a scanning probe, said detector comprising:
a probe having a tip at the front end thereof;
a means for oscillating the probe;
a means for detecting the oscillation of the probe and converting it into an electric signal; and
a means for detecting the signal representing the amount of shift of the resonance frequency of the probe due to the physical interaction between the tip of the probe and the surface of the specimen held in a non-contact state relative to the probe out of the electric signal,
said means for detecting the signal representing the amount of shift of the resonance frequency including:
a frequency changing circuit adapted to receive the electric signal as input and convert it into an electric signal having a lower frequency without changing the signal representing the amount of shift of the resonance frequency; and
a frequency/voltage conversion circuit adapted to receive the electric signal having the lower frequency converted by the frequency changing circuit as input, convert it into a voltage corresponding to the signal representing the amount of shift of the resonance frequency and output the voltage.